The present invention relates to a two-circuit vacuum brake booster which includes two vacuum housings arranged axially behind each other and each provided with at least one wall which is movably arranged therein. The movable walls act on a force-transmitting member and subdivide each of the vacuum housings into a vacuum chamber and a working chamber.
Such a two-circuit vacuum brake booster has come to knowledge from German Pat. No. DE-OS 2,314,792. Each of the movable walls is fastened on a different control hub each of which contains a valve arrangement including a valve piston and a plate valve. The control hubs are designed such that they will penetrate the vacuum and working chambers and will slidingly project from the vacuum housings in an axially sealed manner. The first control hub serves as the air supply to the first valve arrangement and houses the pedal-actuated piston rod which is mechanically connected with the first valve piston. A reaction mechanism adding up the pedal force and the boosting force, which acts on the movable wall, in a predetermined ratio will operate a control rod arranged in the control hub, the control rod acting on the second valve piston in the second control hub. The first control hub slides in a sealed manner in the first vacuum chamber in the second control hub and, thus, a separation of the two vacuum circuits is ensured. A further reaction mechanism will ensure that the boosting force of the second working chamber, together with the force transmitted onto the second valve piston, will act on the force-transmitting member sliding in the second control hub. The support for the reception of the force-transmitting member has a bore which ensures the supply of atmospheric air to the second valve arrangement. Upon actuation of the brake pedal, the axial displacement of the valve piston will provide for the supply of atmospheric air to the first working chamber, the forces that result from the pressure difference moving the first movable wall with the first control hub in the direction of the force-transmitting member. After overcoming of a first dead travel of the first vacuum booster, a piston rod which axially slides in the first control hub will move the second valve piston, atmospheric air also entering the second working chamber due to the air supply from the side of the force-transmitting member. After overcoming of the dead travel of the second vacuum booster, the second movable wall will transmit the forces by which it is acted upon to the force-transmitting member. In this arrangement, the costs are enormous because of the multitude of components. A multitude of seals and springs consume unnecessary energies, while the at times narrow air supply lines tend to rapidly become dirty if the operational surroundings are unfavorable which will result in a derogation of the operation of the vacuum booster.